1. Field of the Invention
The present invention relates generally to recording media, and particularly pertains magnetic recording media which have a thin layer of ferromagnetic metal as a recording layer by which signals such as of video, audio and data are recorded in high density.
2. Description of the Related Art
Recently, a recording medium using ferromagnetic metal such as Co, Ni, Cr and Fe is drawing attention as a recording medium to realize high density magnetic recording, and various investigations have been made to put it to practical use.
In the magnetic recording, when spacing is brought about between a recording layer and a magnetic head as an electromagnetic convertor, spacing loss is caused thereby. Particularly, when the recording frequency is high, such as in a high density recording, the above-mentioned clearance should be made as small as possible, since the spacing loss becomes remarkable when the spacing exists. In recording or playback an audio-video signal, the magnetic head is usually made to contact the recording media. And further, even in recording of a data signal, when the demand for reliability of recording is not so severe, for instance, in case of a floppy disk, the recording media can be made to contact the magnetic head. When the demand for reliability of recording is high, the magnetic head is made to float apart from the surface of the recording media to keep non-contact in order to avoid undesirable friction between the magnetic head and the recording media. However, even in such high reliability multiple magnetic recording systems, they are constructed in a manner that the magnetic head contacts with the recording media at the time of starting or stopping of recording or playback. And further, since the flotation of the magnetic head above the recording medium face causes a spacing loss, they are unsuitable for high density recording. From the above viewpoint, considering the adaptation for high density recording in future, it is more desirable that the recording medium contacts with the magnetic head, thereby to realize and retain the reliability of recording.
That is, a magnetic recording system has a problem that is fundamentally different from that of a optical recording system, because in the magnetic recording system the magnetic head generally contacts the recording medium.
The aforementioned magnetic recording medium, which has thin layer made of ferromagnetic metal such as Co, Cr, Ni and Fe as recording layer, has a large coercive force. Therefore, it is suitable for high density recording, for instance, vertical recording, and so on. However, when a protective layer is not formed on a surface of the ferromagnetic metal thin layer, the formation of an effective protective layer on the recording layer is advantageous to protect the recording layer from damage from contacting with the magnetic head.
In conventional magnetic recording media, wherein magnetic powder mixed with binder is applied to the base film as substrate, so as to avoid problem caused by contact of the magnetic head and the magnetic recording media, a sustance has been added to the binder to give resistance against wear and to impart smoothness to the contact surface of the magnetic recording media. However, in magnetic recording media having a ferromagnetic metal thin layer as a recording layer, such improvements are intended in wear resistance, smoothness and running durability of the magnetic recording media per se as oxidizing surface of the recording layer or the like. In such case, deterioration of the magnetic property of the recording layer can not be avoided. Therefore, it is necessary to keep the characteristics of the recording layer, such as wear resistance, by forming a protective layer on the surface of the recording layer. And further, since such a protective layer brings about distance between the magnetic head and the recording layer, the thickness of the protective layer should be as small as possible.
Hitherto, a protective layer containing an organic substance as a lubricating material and made by vacuum vapor deposition on the surface of the ferromagnetic metal has been developed. But since it has poor wear resistance, it could not bear long time of use. And further, although trials have been made in forming the layer of amorphous carbon or graphite on the surface of the ferromagnetic metal layer by vacuum vapor deposition, spattering or the like, improvement of the smoothness has been obtained only to a certain extent, while sufficient wear resistance could not be obtained.
In case that the above-mentioned material is used as the protective layer of a magnetic recording medium havng a recording layer of a thin layer of ferromagnetic metal, the thickness of the protective layer could not be decreased without losing wear resistance. As a result, the distance from the magnetic head to the magnetic recording medium becomes large, thereby generating a large spacing loss. And further, since the above-mentioned protective layer is worn by contacting with the magnetic head, fine powder of the protective layer material is produced and the powder sticks to the magnetic head, hence remarkablly lowering quality and level of output due to clogging up and dropout.
Owing to the above-mentioned problems, practical use of the high density magnetic recording media having a ferromagnetic metal thin layer as a recording layer has been hitherto much restricted. Therefore, satisfactory high density recording can not be attained so long as the above-mentioned problems are not settled.
To solve the above-mentioned problems, a protective layer which is splendid in smoothness, wear resistance and adhesive ability is indispensable. Furthermore, it is required that: substrate or base material (hereinafter referred to as base material) such as polyethylene based film (in case of tape-shaped magnetic recording media) is not damaged by rising temperature, while the protective layer is formed; a homogenous layer can be formed, even if the thickness of the protect layer is small; and the deposition rate of the protective layer is large, and is suitable for mass production.
It is thought that diamond is suitable for material of the protective layer satisfying the abovementioned requirement. Diamond is a crystalline material having the highest hardness among all substances, it is extremely chemically stable and it is considered to be splendid in the wear resistance and durability in various surroundings of usage. Concerning methods for forming a thin layer of diamond, many reports have been made, and they are disclosed in reference literature as follows:
(1) OYOKIKAIKOGAKU (Applied Mechanical Engineering),
July/1984, "DAIAMONDO HAKUMAKU NO TEIATSUGOSEI NO KENKYU" (Investigation of low pressure synthesis of diamond thin layer)
(2) GENDAIKAGAKU (Modern Chemistry), September/1984, "DAIAMONDO NO TEIATSUGOSEI" (Low pressure synthesis of diamond)
(3) NIHON SANGYOGIJUTSU SHINKOKYOKAI (Japan Industrial Technology Promotion Association) GIJUTSU SHIRYO (Technical data) No. 138, June/1984, "DAIAMONDO NO TEIATSUGOSEI" (Low pressure synthesis of diamond)
However, all of the reports made in them are in an academic researching stage, and have not been put into practice. Further, it is very difficult to use any of these reported methods as a forming means for a diamond protective layer for magnetic recording media, because the above-mentioned methods require heating of the base material to a high temperature (above 400.degree. C.), and the rate of deposition of the diamond layer is slow (a maximum of 200-300 .ANG./minute).